The present invention relates generally to digital demodulation systems for demodulating intermediate frequency (IF) versions of radio frequency signals carried respectively on orthogonally oriented polarization planes, and more specifically to cancellation of interference between the polarization planes.
To achieve frequency utilization it has been a common practice to transmit two digital signals on respective channels formed by orthogonally oriented polarization planes, either vertical-horizontal or clockwise-counterclockwise orientation. However, fading and anisotropic nature of the transmission medium are the potential source of troubles, causing cross-polarization interference between the orthogonal planes. As shown and described in U.S. Pat. No. 4,575,862 issued to M. Tahara et al and assigned to the same assignee as the present invention and a paper "IF Band Cross-Polarization Canceler" by T. Ryu, M. Tahara and T. Noguchi, IEEE International Conference on Communications, Proceedings, Volume 2 May 14-17, 1984, Amsterdam, prior art cross-polarization cancellation involves the use of a single digital demodulator for each channel to process the intermediate frequency (IF) version of a radio frequency signal. The demodulator includes an (N+1)-bit analog-to-digital converter. The higher significant N bits of the (N+1)-bit output of the converter represent the original N-bit code and the LSB (least significant bit) of the (N+1)-bit converter output, which represents the cross-polarization interference, is supplied as an error component to a correlator of the own channel, while the MSB (most significant bit) of the (N+1)-bit converter output is cross-coupled to the correlator of the other channel as a data component. The correlator of each channel generates a control signal with which the amplitude of the IF signal of the other channel is proportioned and subtractively combined with the IF signal of the own channel, the combined IF signals being fed to the digital demodulator of the own channel to cancel the cross-polarization interference contained in the IF signal of the own channel. Since each demodulator forms part of the closed loop of the other channel, a modification of the operating parameters of one results in an alteration of the operating parameters of the other channel equipment.